Super Earths: Massive rocky worlds that redefine habitability, challenge life

Super Earth Exoplanet are worlds in orbits around distant stars that have rocky compositions just like the Earth, but are larger than the Earth. These worlds can be twice as wide as the Earth, containing up to twice the mass. Larger worlds are considered Sub Neptunes. While there are no Super Earths in the Solar System, such planets are common in the galaxy. These worlds have a stronger gravity, which means that they are likely to have flatter surfaces, with fewer tall mountains that would get eroded easily. These Super Earths can also more easily retain their atmospheres because of the stronger gravity, which is conducive for liquids on the surface. If the Super Earths are in habitable zones around their host stars, they may be covered by archipelagos.

The discovery of Super Earths in the Milky Way has reshaped our understanding of planet assembly, composition and potential habitability. With larger surface areas, Super Earths may just be superhabitable as well. The densities of Super Earths can very widely, reflecting their diverse compositions. Dense Super Earths such as Kepler-10 b are likely to be rocky with iron-rich cores and silicate mantles. Planets with a lower density such as GJ 1214 b can have hydrogen-helium envelopes with volatile-rich compositions. If these are ocean worlds with hydrogen-rich atmospheres, then they are called Hycean worlds. The mass-radius relationship is a crucial diagnostic tool. For a given mass, radius depends on the composition.

Rocky Super Earths follow a roughly linear mass-radius scaling, from which the planets rich in volatiles tend to deviate. These icy worlds have larger planetary radii because of extended atmospheres. Models of the interior of such worlds based on equations of state for silicates, iron and volatiles predict the core-mantle structures of Super Earths. High-pressure phases of water or hydrogen may dominate in water-rich planets or those with extended atmospheres. These worlds have core mass fractions similar to that of the Earth.

Super Earths are assembled in protoplanetary disks, the leftover material swirling around newborn stars through two processes, core accretion or disk instability. Small particles clump together into pebbles, boulders then mountains to form planetary embryos, accreting gas if they reach a critical mass of around 10 Earth masses before the disk dissipates. Super Earths fail to accrete massive gas envelopes, that prevents them from becoming gas giants. Migration also plays a crucial role with many Super Earths residing in tight orbits around their host stars, with years less than 100 Earth days long. There is not sufficient material in the protoplanetary disks to form such large worlds at such close distances, which indicates that these planets have drifted inwards. Type I migration, where planets interact gravitationally with each other within the disk can trap Super Earths in resonant chains, as seen in systems such as TRAPPIST-1, which has seven terrestrial exoplanets all orbiting closer to the host star than Mercury is to the Sun.

Post-formation evolution further shapes the properties of these worlds. The Super Earths in tight orbits face intense stellar radiation, that strips them of their atmospheres. Hydrodynamic escape models indicate that low-mass planets with hydrogen-rich envelopes can lose significant mass, potentially transforming Sub Neptunes into Super Earths. Tidal heating from orbital eccentricity or spin-orbit resonances can further influence internal dynamics, potentially sustaining geological activity. Geological activity as well as archipelago conditions, with extended coastlines increase the habitability of exoplanets.

Super Earth atmospheres can vary from wispy carbon dioxide or nitrogen dominated layers to thick hydrogen-helium atmospheres. The composition of the atmospheres depends on the formation history and the amount of stellar irradiation. Rocky Super Earths with secondary atmospheres that are outgassed from the mantle can contain carbon dioxide, water and methane, depending on the temperatures. Volatile rich Super Earths are likely to retain primordial and pristine hydrogen-helium envelopes, with trace amounts of methane and ammonia.

The James Webb Space Telescope with its transmission spectroscopy is providing scientists with a window into the skies of distant worlds, revealing the properties and compositions of their atmospheres. GJ 1214 b has a flat transmission spectrum indicating high-altitude clouds or hazes that are obscuring molecular features. Climate models for tidally locked Super Earths predict strong contrasts between day and night side temperatures, resulting in strong winds circling the planets. These worlds have searing hot daysides and cool nightsides, where conditions support the condensation of volatiles.

The habitability of Super Earths hinges on the surface conditions, stability of the atmspheres and the stellar environment. Planets in the habitable zone, where liquid water can exist are prime targets for the search for alien life. However, while some factors make conditions favourable to life, others do not. The high surface gravity increases the atmospheric scale height, trapping more heat. Tidal locking induces extreme climate gradients. Surface gravity may also limit tectonic activity, reducing the ability of the world to cycle volatiles, which is critical for habitability.

The TRAPPIST-1 system contains water-rich super Earths that are promising candidates for harbouring lifeforms. Models indicate that planets with 10 to 50 per cent water by mass could host deep oceans, buffered by layers of high-pressure ice. Runaway greenhouse effects or atmospheric loss can complicate habitability. Biosignatures such as oxygen, methane and nitrous oxide can be detected with spectroscopy, but these signals can be mimicked by abiotic processes as well.

Super Earths bridge the gap between terrestrial and gaseous planets, and are expected to be common across the galaxy. These worlds are prime targets for the search for extraterrestrial life, but such life may live under challenging conditions. The higher gravity means that these lifeforms may perceive time more slowly, and the energy cost of locomotion would be much higher. Creatures similar to fleas, kangaroos or grasshoppers would find it far more challenging to jump. If there are intelligent, advanced species on these worlds, they may never be able to escape the crushing gravity of the planet. To escape the surface and access space, the inhabitants would require rockets four to five times the height of the pyramids, on a Super Earth containing ten times the mass of the Earth. All of this goes to reinforce just how lucky humans are, and why Earth is perfectly sized to harbour life.